1. Field of the Invention
The present invention relates to a measuring method for a bearing cup gap in a universal joint installed in, for example, a vehicle, and a method for manufacturing a universal joint to which the measuring method is applied.
2. Description of Related Art
For example, as a universal joint used for a drive shaft, a steering apparatus, or the like in a vehicle, a universal joint described in Japanese Patent Application Publication No. H7-233820 (JP H7-233820 A) is known. The universal joint described in JP H7-233820 A includes: a joint spider member 12 with four shafts 12a (shaft portions) arranged in the shape of a cross; and bottomed tube-like bearing cups 14 installed so as to be rotatable relative to the shafts. The bearing cups 14 are press-fitted into bearing holes having the same axis and formed in respective arms of a pair of arms extending in a bifurcated manner from a yoke main body portion of a yoke member such that the openings of the bearing cups 14 face each other. The coaxially arranged shafts 12a of the joint spider member 12 are each fitted in the corresponding bearing cup 14 via a resin plate.
In a technique described in JP H7-233820 A, when the bearing cups 14 are assembled in the bearing holes in the arms, first, an initial state is established where the shaft centers of the shafts 12a have been inserted into the bearing cups 14 and centered with respect to the yoke member. Then, the first bearing cup 14 is press-fitted into the bearing hole while the second bearing cup 14 is fixed. At this time, a jig with a strain gauge is attached to the joint spider member 12. The first bearing cup 14 is pushed and press-fitted in a direction from the first bearing cup toward the second bearing cup (hereinafter simply referred to as a second direction). Thus, an inner bottom surface of the first bearing cup 14 presses an end surface of the shaft 12a via the resin plate to allow the joint spider member 12 to start moving in the second direction. The moment of the movement can be detected based on an increase in output from the strain gauge of the jig. Then, the second shaft 12a moves in the second direction inside the second bearing cup 14 to eliminate the gap between: the second shaft 12a; and the resin plate and the bottom surface. The second shaft 12a further continues moving to compress the resin plate. A reaction force to the compression causes the second bearing cup 14 to press the second shaft 12a. 
Then, the first bearing cup 14 is fixed, and the second bearing cup 14 is press-fitted into the bearing hole in the arm. The press fitting is continued until the output from the jig with the strain gauge returns to zero again. When the output reaches zero, the press fitting is ended. Thus, the position of the joint spider member 12 returns to an initial state where the joint spider member 12 is centered with respect to the yoke member again. At this time, the joint spider member 12 is subjected to even reaction forces to compressive loads, from the resin plates in the first and second bearing cups.
However, in the above-described conventional technique, the press fitting load applied when the bearing cup 14 is press-fitted into the bearing hole is not necessarily the same for the first bearing cup and for the second bearing cup. Consequently, during the press fitting of the bearing cups 14, a strain amount (displacement amount) of the joint spider member 12 in a press fitting direction may be different for the two arms. Therefore, when the press fitting load is released, the arms may be sprung back by a distance corresponding to the applied press fitting load, the shape and rigidity of each arm, and the like, resulting in a difference in size between gaps (bearing cup gaps) formed in the first bearing cup and in the second bearing cup. Thus, when the backlash is eliminated using snap rings selected on the assumption that the gap in the first bearing cup and the gap in the second bearing cup are equal, to regulate movement of the joint spider member 12 with respect to the yoke member, the shaft center of the joint spider member 12 may be displaced with respect to the yoke member.